The present invention generally relates to the field of radio frequency (RF) power amplifiers. More specifically, it relates to Complementary Metal Oxide Semiconductor (CMOS) power control circuits used in the Global System for Mobile (GSM) communication power amplifier modules.
GSM is a digital mobile telephony system used for cellular communication. The GSM technology uses Gaussian Minimum Shift Keying (GMSK) which is a continuous-phase frequency-shift keying (FSK) modulation. GMSK entails passing a modulating signal through a Gaussian filter, subsequent to which a carrier signal is modulated using the filtered modulating signal. Modulating the carrier signal includes varying the frequency of the carrier signal at carrier zero crossing points such that the GMSK modulated carrier signal has a continuous phase and a constant amplitude envelope.
RF power amplifiers which include non-linear power amplifiers are used to amplify the GMSK modulated carrier signal. The amplification provided by such RF power amplifiers is proportional to the amplitude of an input signal. This results in a distorted output signal in a scenario when the amplitude of the input signal varies. Thus, the constant amplitude envelope of the GMSK modulated signals renders such signals immune to distortion due to non-linear amplification. Therefore, RF amplifiers may be used for power control of GMSK modulated signals without distorting the signals. Additionally, the constant amplitude envelope of the GMSK modulated signals permits the RF power amplifiers to operate near saturation, which in turn results in energy efficiency. The energy efficient operation of the RF amplifiers enables reduction in the size of mobile hand set and minimization of the power consumption to conserve life of the battery used in the mobile handset.
The RF output signal is composed of multiple signals having different frequencies spread over a frequency spectrum. In accordance with European Telecommunication Standard Institute (ETSI) specifications, the frequency band of the RF output signal should be limited to a predefined frequency range or have a predefined switching spectrum. The frequency range of the RF output signal spectrum depends upon the operation of the RF power amplifier which in turn is governed by a biasing voltage. Thus, to limit the RF output signal spectrum to a predefined range, a power control circuit is used to control the biasing voltage provided to the RF power amplifier. The operation of the power control circuit is explained below in conjunction with FIG. 1.
FIG. 1 is a schematic representation of a conventional power amplifier module 100. Power amplifier module 100 includes an RF power amplifier block 102 and a power control circuit 104. RF power amplifier block 102 includes one or more power amplifiers. Power control circuit 104 includes an amplifier 106, a pass transistor 108, and a voltage source 110. An analog ramp voltage (Vramp) is applied at a negative terminal of amplifier 106.
An output terminal of amplifier 106 is coupled to a gate terminal of pass transistor 108. A source terminal of pass transistor 108 is coupled to voltage source 110. Voltage source 110 provides a voltage (Vbat) to the source terminal of pass transistor 108 having a maximum magnitude equal to a second voltage level when voltage source 110 is completely charged. A drain terminal of pass transistor 108 is coupled to RF power amplifier block 102. Further, an output voltage (Vout) is obtained at the drain terminal of pass transistor 108. Vout is provided as biasing voltage to the one or more power amplifiers of RF power amplifier block 102.
Power control circuit 104 includes resistor R1 through which Vout is fed back as a feedback voltage to a positive terminal of amplifier 106. A first end of resistor R1 is connected to the drain terminal of pass transistor 108. A second end of resistor R1 is connected to the positive terminal of amplifier 106 and a first end of resistor R2. A second end of resistor R2 is coupled to a first end of resistor R3 and a first end of resistor R4. A second end of resistor R4 is connected to ground.
RF power amplifier block 102 amplifies an input RF signal to produce an output RF signal in which the power of the output RF signal is controlled by RF power amplifier block 102. Power control circuit 104 controls the biasing voltage Vout which is provided to the one or more power amplifiers of RF power amplifier block 102. The control of the biasing voltage Vout in turn controls the power of the output RF signal. Further, power control circuit 104 operates as a low drop-out voltage regulator (LDO). The operation of the LDO is based on a feedback loop, which includes feeding back Vout to the positive terminal of amplifier 106. Amplifier 106 provides an amplified error voltage at the output terminal by amplifying a difference between the feedback voltage applied at the positive terminal of amplifier 106 and Vramp applied at the negative terminal of amplifier 106. The amplified error voltage is applied to the gate terminal of pass transistor 108 and has a magnitude equal to Vgpass. The feedback voltage depends on Vout and is equal to a voltage at the second end of resistor R1. Vout depends on the amplified error voltage. For a predefined profile and magnitude of Vramp applied at the negative terminal of amplifier 106, power control circuit 104 regulates Vout at the drain terminal of pass transistor 108. As a result of which power at the output of RF amplifier block 102 is controlled.
Power at the output of RF amplifier block 102 is constant under normal operating conditions for an applied voltage Vramp during which the magnitude of Vout is sufficiently lower than Vbat. However, as voltage source 110 discharges, Vbat reduces below the maximum magnitude, i.e., the second voltage level, and drops below the Vout voltage levels for high power settings. The gain in the error amplifier 106 then decreases since Vout exceeds Vbat. Also, the voltage drop between Vbat and Vout decreases, causing pass transistor 108 to operate in the triode region. As a result, the unity gain frequency and loop gain of power control circuit 104 degrades and hence the transfer function characteristics of power control circuit 104 become distorted. As a consequence, Vout applied to RF power amplifier block 102 is distorted. In addition, Vgpass drops significantly due to the decrease in gain of amplifier 106. This introduces slewing transients and delay in the feedback loop of power control circuit 104 when more current is required to recharge parasitic capacitances at the gate terminal of pass transistor 108. Further, the reduced Vbat prevents Vout from reaching its maximum value, causing clipping of the ramp voltage profile at output of amplifier 106. As a result, spurious non linear components are introduced in the output of amplifier 106 which degrades Vout applied to RF power amplifier block 102.
Due to the reduction in Vbat, the slewing transients and delays are introduced in the feedback loop of power control circuit 104 and due to the variations in Vout, spurious frequency components are introduced in the frequency spectrum of the RF output signal. This degrades the RF output signal frequency spectrum and causes the RF output signal to violate the frequency requirements defined by the ETSI.
In light of the above, a need exists for a power control circuit that regulates an output voltage applied to RF power amplifier block. Further, a need exists for a power control circuit that prevents the transfer function characteristics of the power control circuit from becoming distorted, thereby maintaining the required switching spectrum when the magnitude of the voltage provided by the voltage source reduces. Still further, a need exists for a power control circuit that can reduce the effect of slewing due to large capacitance at the gate terminal of pass transistor and prevents the non-linear operation of amplifier, thereby preventing degradation of the switching spectrum. Furthermore, there is a need for a power control circuit that reduces clipping of the applied ramp voltage profile at reduced voltage level of the voltage source.